Linear servo accelerometer systems and various circuits and mechanical arrangements associated therewith are well known. Such systems are generally described only so that the general environment of the subject invention may be understood.
Such systems may, for example, include means to permit a movable mass to be freely moved in a pivot assembly in response to acceleration forces. The movable mass may be connected to a torque coil disposed in a magnetic field In some case, the torque coil itself may comprise the seismic element.
Position detector means, not forming a direct part of the subject invention, are generally employed with servo amplifiers of the type involving the subject invention. A position sensor is used to generate a signal corresponding to the position of the seismic element. This signal is generally applied to an input circuit of an amplifier included in a servo loop. The output signal from the servo amplifier, which could be considered an error signal, is applied to a torque coil which tends to force back the coil to the position it was at before acceleration forces were applied.
One type of position sensor could be an oscillator in which a conductive element, which may be the seismic element, is coupled to the frequency determining tank circuit of the oscillator. When the conductive element is moved, the amplitude of the oscillator is correspondingly varied. A rectifier circuit may be provided for rectifying the output of the oscillator. The torque coil is generally physically connected to the conductive element. The output signal from the rectifying circuit is connected to the torque coil and in effect is used to oppose the acceleration forces applied to the conducting element by restoring the torque coil to a null position.
The general amplifier used in the subject invention is similar in many respects to the amplifier described in a copending application filed concurrently herewith entitled "Servo Amplifier for an Electrically Damped Accelerometer", Ser. No. 905,012, filed May 11, 1978. The amplifiier described in this copending application emphasizes advantages which are also found in this application. However, this application involves an improved output circuit involving the torque coil which facilitates testing and not subject to variable load impedances.
In the copending application to which reference is made, as well as many prior art circuits, the torque coil in the output circuit is often connected to a resistive or other reactive element. As a result, if it is desired to test the output response of the mechanical system by applying a test current through the torque coil, the impedances associated with the torque coil will tend to vary because of the different electrical components associated with the coil but not forming part of the mechanical arrangement of the accelerator under test. Thus, it is difficult to sample test one accelerometer unit with the assurance that other units will test the same way with distortions caused by impedance changes resulting from slightly different valued elements associated with the torque coil.
It is an object of this invention to provide an improved servo amplifier for an accelerometer system having a low output impedance with improved means for testing the current through a torque coil.
It is a further object of this invention to provide an improved servo amplifier system for an accelerometer system in which the frequency characteristic of the circuit including the coil for measuring acceleration forces is minimized.
In accordance with the present invention, a servo amplifier system for producing error signals for application to a torque coil to counteract the movements resulting from acceleration forces is provided. The system includes first and second operational amplifiers. Electrical signals from a position detector are applied from the output circuit to the input circuit of the first amplifier. The second amplifier has a low impedance output circuit. A torque coil connects the output of the first amplifier to the input of the second amplifier free of any load impedance. A feedback network is connected from the output circuit of the second amplifier to the input circuit of the first amplifier. A utilization device may be connected to the output circuit of the second amplifier.